Radio communication systems are known in which a communication unit, such as a mobile or a portable radio, wirelessly communicates with a fixed infrastructure comprising multiple geographically-diverse base sites. Each base site comprises one or more base stations along with other items of equipment. In the design of such a system, the base sites are geographically located in such a way as to minimize the likelihood that wireless communications between a communication unit and a base site will degrade below a desired minimum level of signal quality.
As a communication unit roams throughout the radio communication system, a system controller, such as a base site controller, assigns a base site to serve the communication unit. In selecting a base site, the system controller analyzes a variety of factors, such as the downlink carrier-to-interference plus noise ratio (C/I+N) of the signals received by the communication unit from several base sites and the uplink C/I+N of the signals received by the base sites from the communication unit. The system controller then typically assigns the base site yielding the best uplink and downlink C/I+N to serve the communication unit, assuming the base site has communication channels available. As the communication unit roams from the service coverage area of one base site to the service coverage area of another base site, the system controller typically arranges for a "hand off" of communication service from the currently serving base site to the base site providing communication service to the service coverage area that the communication unit is entering once the C/I+N for the former base site degrades below a preset threshold and the C/I+N for the latter base site is at or above the preset threshold with respect to the communication unit. This handoff must be seamless and transparent to the user of the communication unit (i.e., the communications must appear to continue uninterrupted and no information can be lost as communication service is transferred among base sites).
In a time division multiple access (TDMA) radio communication system, information is transmitted within assigned time slots to and from each communication unit. For proper operation, all of the communication units and base sites in the system must be synchronized in time. While communication units automatically synchronize to the base sites, all base sites must be accurately synchronized to each other to allow a communication unit to handoff between base sites without a loss of information.
Base site time synchronization is also used to control the time of launch of a location beacon that the communication units can use to measure time differences of arrival of the beacon signal from the base sites. Time difference of arrival information from at least three sites, along with base site position information, can be used to calculate a communication unit's location.
One method of time synchronization of the base sites' timing references is to connect the base sites, via T1 or E1 links, to a master synchronization unit that generates a common time base. However, highly accurate time synchronization cannot be reliably obtained by sending a signal over T1/E1 links due to differences in inbound and outbound delays when a T1/E1 link that interconnects two base sites, but is not dedicated to time synchronization, is routed through a switch (i.e., when using a T1 /E1 link, the inbound signals may travel a different path from the outbound signals and will incur different propagation delays).
A second, more accurate method of time synchronization uses a global positioning system (GPS) satellite and GPS receivers as a source of a common time base. All base sites within the coverage area of a GPS satellite can synchronize to a highly stable and accurate GPS signal from the GPS satellite. However, GPS synchronization is not an available option for base sites that are not within the coverage area of a GPS satellite, such as base sites located deep in a tunnel or base sites located in the interior of a large office building and which lack access to a roof top antenna.
Once a base site is synchronized, it is able to maintain the timing reference by use of a local oscillator. Any lack of stability in the oscillator can be periodically corrected by resynchronizing with the GPS signal or through periodic corrections of the oscillator based on known drift in the oscillator performance. In the latter case, a base site might still occasionally have to resynchronize with a common time base. However, if a base site shuts down completely, for example due to a power outage or for maintenance purposes, it will need to be time synchronized when powered back up. If such a base site is not within the coverage area of a GPS satellite, it can be manually synchronized by bringing a high stability clock, such as a rubidium clock, to the site. However, manual synchronization will not work if the site is being remotely restarted, and is not practical if such a site needs to periodically resynchronize by reference to a common time base.
Therefore, a need exists for a method and apparatus for performing a highly accurate time synchronization of a base site that is not within the coverage area of a GPS satellite.